In a time-of-flight mass spectrometer (TOF-MS), ions separated by a time-of-flight mass analyzer according to their mass-to-charge ratios m/z arrive at an ion detector, whereupon the ion detector produces a detection signal having a peak whose intensity corresponds to the number of ions which have arrived at the detector. A data processing system for a TOF-MS processes this detection signal to create a time-of-flight spectrum with the abscissa showing time and the ordinate showing signal intensity, and converts the time of flight into mass-to-charge ratio to create a mass spectrum. To obtain a correct mass spectrum, and to avoid selecting an inappropriate peak when automatically selecting a precursor ion based on the mass spectrum and performing an MSn analysis, it is desirable that the data processing system should eliminate the largest possible portion of the noise components, which occur due to various factors and overlap the detection signal, before the creation of the time-of-flight mass spectrum.
Normally, the intensity (amplitude) of the ringing component accompanying an objective peak is adequately lower than the signal intensity of the objective peak and continues for only a short period of time. Taking this into account, in a mass spectrometric data processing system disclosed in Patent Document 1, a simple method has been proposed in which a certain threshold for removing noise components is set for the detection signal and any signal value lower than the threshold is replaced with a predetermined value (e.g. a value on the baseline) to eliminate noises inclusive of the ringing.
This method can be used without problem if all the signal peaks have adequately high intensities. However, if a peak with low signal intensity is mixed, it is difficult to set a threshold which correctly distinguishes between the noise and the peak. An incorrect setting of the threshold may possibly eliminate a peak with low signal intensity, or conversely, allow a relatively high-intensity ringing which accompanies a peak with high signal intensity to remain. In recent years, an analysis of substances with extremely low concentrations has been increasingly important in the field of mass spectrometry, and the aforementioned ringing elimination method, which lowers the analyzing sensitivity or signal-to-noise ratio, is not so appropriate. Furthermore, in the case of using multiply-charged ions for the measurement of high-molecular compounds, the period of time from one peak to the next is so short that the next peak is most likely to overlap the period of ringing.
Unlike the simple aforementioned procedure, a method disclosed in Patent Document 2 uses a more complex process to analytically deduce the waveform of a ringing in an input signal and eliminate the ringing. A more detailed description of the process disclosed in the aforementioned document is as follows: An input signal is subjected to an up-sampling process. The resultant signal is divided into higher and lower frequency components, and the peak value of the higher-frequency component is determined. From this peak value and the fluctuation of the lower-frequency signal at the point of occurrence of the peak value, a coefficient corresponding to the amount of ringing is determined. From this coefficient and the higher-frequency component of the input signal, the ringing waveform is deduced.
However, in the ringing elimination method described in Patent Document 2, the deduction accuracy of the ringing waveform is not very high under some conditions, e.g.
when the S/N ratio of the signal during the time region where the ringing is present is low, or when the frequency of the ringing is relatively high with respect to the sampling frequency for the analogue-to-digital conversion of the signal and the number of sample points in one cycle of the ringing waveform is small (e.g. five to ten points). Therefore, this method is not suitable for eliminating the ringing in a TOF-MS or similar mass spectrometer which has a high mass-resolving power and is often operated under the aforementioned disadvantageous conditions.